1. Field of the Invention
The present invention relates to a chlor-alkali cell, and more particularly to a chlorine gas generator unit for on-site usage of the chlorine gas as a disinfectant within swimming pools or other aquatic apparatus involving the storage and recycling of water.
2. Description of the Prior Art
There are several methods of treating water to kill living organisms, particularly pathogenic bateria. The application of chlorine or chlorine compounds is the most common, although less frequently used methods include the use of ultraviolet light, ozone, or silver ions.
For large systems, chlorine gas is the most economical and easiest to apply. It is typically handled in liquid form in pressurized tanks, and introduced either directly into the receiving water through diffusers, or by first being dissolved into a separate stream which is then discharged into the receiving water.
For emergency uses, or where water consumption is small and the chemical cost is of no significance, hypochlorination is employed. Hypochlorination used calcium or sodium hypochlorite as the disinfectant, the former being a solid and the latter, a solution. Although the equipment required for handling the hypochlorites is less expensive than that necessary for free chlorine, the cost of the chemicals is greater.
A further application technique involving treatment with chlorine gas utilizes a chlorine gas generator for the on-site production of the gas. There are several methods of obtaining chlorine gas, and most of them involve the electrolysis of a chlor-alkali solution.
The nascent electrolytic production of chlorine gas from an electrically conducting solution which contains chloride is facilitated by applying a current across a submerged positively charged anode. The electrolyte will also contain a negatively charged cathode to complete the circuit. The chloride anions present within the electrolyte are oxidized to produce chlorine gas. An energy source of direct current or of a rectified alternating current is used to provide the necessary electrical potential.
Parallel with the production of chlorine gas at the anode, cationic hydronium ions present from dissociated water are reduced at the negatively charged cathode and result in the generation of hydrogen gas. As the water is dissociated into the components of hydronium cations and hydroxide anions, and with the further transformation of hydronium ions into hydrogen gas, there is the ensuing production of an aqueous caustic or hydroxide solution at the cathode.
The electrode used in the generation of chlorine gas, (the anode), and the electrode used in the generation of hydrogen gas and resultantly hydroxide, (the cathode), can be either dimensionally stable or non-dimensionally stable. A dimensionally stable electrode, one that does not wear away with use, can consist of the following: titanium oxide and ruthenium oxide coated substrates (U.S. Pat. No. 3,963,593), a plantinized titanium material (U.S. Pat. No. 3,963,592), the silicade of platinum group metals (U.S. Pat. No. 3,963,593), a boride of either titanium, tantalum, zirconium, aluminum, hafnium, niobium, tungsten, yttrium, molybdenum, or vanadium (U.S. Pat. Nos. 4,111,765, 4,055,477), a substrate coated with the oxide of hafnium (U.S. Pat. No. 4,012,296), a bronze substrate coated with platinum, irridium, rhodium, palladium, ruthenium, or osmium (U.S. Pat. No. 4,032,417), cobalt spinels (U.S. Pat. No. 4,061,549), yttrium oxide matrix (U.S. Pat. No. 4,098,669), titanium or tantalum substrate coated with platinum and doped with the oxide of either silver, tine, chromium, lanthanum, cobalt, antimony, molybdenum, nickel, iron, tungsten, vanadium, phosphorus, boron, beryllium, sodium, calcium, strontium, lead, copper, or bismuth (U.S. Pat. Nos. 4,070,504, 3,986,942), a lanthanum substrate with a pervoskite surface (U.S. Pat. No. 4,076,611), or a substrate coated with platinum or a noble metal alloy or oxide (U.S. Pat. No. 4,100,050), gold, or treated graphite. The nondimentionally stable anodes are constructed of rods of steel or of graphite carbon (U.S. Pat. No. 4,097,356).
Concurrent with the nascent generation of chlorine gas from chloride solutions, the cathode is usually dimensionally stable and can be constructed of such materials as: a ferrous substrate coated with either tungsten, cobalt, nickel or phosphorus (U.S. Pat. Nos. 4,010,085, 4,086,149), metal carbides, borides, nitrides and sulfides (U.S. Pat. No. 4,098,669), a copper substrate with a coating of nickel, vanadium or molybdenum (U.S. Pat. No. 4,033,837), high surface-area nickel coated steel (see, Chemical Engineering, 87(3):106 (1980), graphite, or stainless steel. The materials of which the cathodes and anodes are fabricated cannot consist of the same material as this will cause excessive electrode wear.
The apparatus used to electrolytically generate chlorine gas, hydrogen gas, and the residual hydroxide solution is termed a "Chlor-alkali cell" (see, Chemical Engineering, 85(16):106 (1978). The three types of chlor-alkali cells are: the mercury amalgam cell, the asbestos diaphragm cell, and the membrane cell. The inherent design of chlor-alkali cell is established on the basis of optimizing the generation of chlorine gas, generating a relatively pure solution of hyroxide (caustic) exclusive of chlorides, minimizing the conversion of chlorine gas to chlorine oxides, and maintaining a separation of chlorine and hydrogen gasses to minimize the loss of chlorine gas via chemical reaction to hydrochloric gas. The separation of the anode compartment-anolyte, and the cathode compartment-catholyte can optimize these operating conditions. This separation has been achieved by chemical and physical means using, respectively, physiochemical solubility or a physical barrier.
The amalgam used in the mercury cell is a free flowing cathode which is relatively insoluble within the solution of the cell and is withdrawn from the cell as a reduced sodium mercury compound (U.S. Pat. No. 3,793,164). The chlorine gas is generated from an anode and the hydroxide and hydrogen gas are generated by combining water with the withdrawn, reduced analgam. The anolyte solution containing the chlorides is separated from the free flowing catholyte by the use of solubility physiochemistry.
The diaphragm which is used to partition a diaphragm cell divides the anolyte and catholyte and retains a cation-permeable nature such that anions such as hydroxide cannot diffuse from the cathode to the anode. This diffusion would result in the formation of chlorine oxides, and loss of hydroxide concentration. The diaphragm partition (U.S. Pat. Nos. 4,121,990, 4,013,525) used in chlor-alkali cells may consist of: a vacuum deposited asbestos species such as crocidotite or chrysolite (U.S. Pat. No. 4,093,533); a synthetic diaphragm of fluorocarbon resins (U.S. Pat. Nos. 3,853,720, 4,118,308); polymeric resins (U.S. Pat. No. 3,775,272); a carbon diaphragm (U.S. Pat. No. 3,223,242); a diaphragm consisting of sand placed with polyarylene sulfide binders (U.S. Pat. No. 4,080,270); chlorotrifluoroethylene fiber materials (U.S. Pat. Nos. 4,126,535, 4,126,536); an ion-exchange membrane with a graft copolymer of polyolefins and hydroxystyrene (U.S. Pat. Nos. 4,025,401, 4,011,147); an asbestos diaphragm composed of sulfonated or halogenated copolymers of styrene and divinyl benzene (U.S. Pat. No. 4,056,447); a ceramic partition with a coating of either Sb.sub.2 O.sub.5, Bi.sub.2 O.sub.5, MoO.sub.3, WO.sub.3, or V.sub.2 O.sub.5 (U.S. Pat. No. 4,119,507), placed on the anode side; or abestos doped with ethylene chlorotrifluoroethylene (Halar) binders (see, Chemical Engineering, 81(4):84 (1974).
An advanced means of compartmentalizing the electrolytic unit which generates chlorine gas, hydrogen gas, and hydroxide is by use of a membrane partition. In the membrane cell the anolyte and catholyte are segregated using a synthetic microporous copolymer which is permselective, hydraulically impermeable, and utilizes inherent anionic characteristics to inhibit the migration of hydroxide ions from the cathode to the anode by repulsion due to like charges. (U.S. Pat. Nos. 4,069,128, 3,773,634, 4,075,068, 3,117,066, 4,080,270, 4,025,405, 4,057,474, 4,111,780, 4,036,714, 4,055,476, 4,056,448). The membrane materials used to conduct a charge yet limit the movement of the hydroxide ions in the chlor-alkali cell consist of: copolymers of divinyl benzene and olefinic carboxylic acids (U.S. Pat. No. 2,731,408); polymerized perfluorosulfonic acid-DuPont's Nafion series (U.S. Pat. Nos. 4,030,988, 4,021,327, 4,056,448, 4,085,071); a polymer of perfluorinated hydrocarbons with side chains of sulfonated perfluorovinyl ether and sulfostyrenated perflourinated ethylene (U.S. Pat. Nos. 4,061,550, 4,062,743); copolymers of perfluoroalkyl or trifluoromethyl subgroups (U.S. Pat. No. 4,080,270); a copolymer of tetrafluoroethylene and sulfonated perfluorovinyl ether (U.S. Pat. Nos. 3,948,737, 3,951,766); and a perflourocarboxylic acid ion-exchange material (Asahi Chemical Industry Co.). These membranes are stable, cationic-permeable derivatives which are electrically conductive. ("Perfluorinated Ion Exchange Membrane", W. T. F. Grot et al. Presented to the 141st National Meeting of the Electrochemical Society in Houston, Texas, May 7-11, 1972; Asahi Chemical Industry Co., 1-2, 1 Chome, Yurakucho, Chiyoda-ku, Tokyo, Japan).
In the operational application of the membrane cell, various constituents such as precipitated compounds, electrode debris, and suspended solids in the make-up water, tend to plug the membranes and render them inoperable or significantly decrease conductive capacities, resulting in an excessively inefficient electrolytic cell. The methods which are utilized to alleviate this problem of a plugged membrane include: the use of phosphate type additives in the cell which form a pH controlled insoluble gel above pH 5.5 (U.S. Pat. No. 3,793,163); the use of chelation via ethylenediaminetetraacetic acid (EDTA); propyleneglycol, or dextrose (U.S. Pat. No. 3,971,706); the intermittent use of acids (U.S. Pat. No. 4,040,919); an intermittent cell pH of 3 to 5 (U.S. Pat. Nos. 3,948,737, 4,055,475); and a specific method of regenerating perfluorocarboxylic acid membranes (U.S. Pat. No. 4,115,240).
An additional problem associated with the use of a membrane in the chlor-alkali cell is physical stress due to high temperatures, the presence of chlorine (an oxidizing environment), and swelling of the membrane from water hydration. These difficulties result in the stretching, shrinking and, warping of the membrane such that the membrane under these conditions causes electromotive shorts and result in an inefficient cell. Hence, to circumvent the deleterious membrane alterations which occur during the operation of a chlor-alkali cell, a method of pre-treatment and pre-processing was devised (U.S. Pat. No. 3,985,631).
In order for a chlor-alkali cell to generate chlorine gas, a source of chlorides must be present in the anolyte which, when oxidized, forms chlorine gas. The most common sources of chlorides used in the electrolyte are sodium chloride (table salt), which is granulated or in the form of rock salt (U.S. Pat. Nos. 3,933,603, 3,773,634, 4,025,405, 4,056,448), or hydrochloric acid (U.S. Pat. Nos. 3,117,066, 3,351,542). In any event, a source of anionic chloride must be provided for the generation of chlorine gas, and the source of chlorides can actually be provided from any one of numerous chloride salts (U.S. Pat. No. 3,361,663).
A by-product of the chlor-alkali cell, which is at times undesireable, is hydrogen gas. Within certain types of electrolytic chlorine gas units, where the generated hydrogen is neither released (vented) to the air nor dissipated in an aqueous body, this possibly explosive gas must be reacted to minimize any potential hazard. The means which has been applied to oxidize this hydrogen gas has been the use of oxygen or air (U.S. Pat. Nos. 3,291,708, B 361,744, 3,941,667, 4,035,254, 4,035,255, 4,121,990) directly within the cell or within an additional piece of equipment.
Most chlor-alkali cells consist of two compartments, namely the anolyte for chlorine generation and the catholyte for hydroxide and hydrogen gas generation. Several recent designs incorporate a third compartment between the anolyte and catholyte and this "buffer compartment", as it is named, is the point of addition of the electrolyte chemicals and water (U.S. Pat. Nos. 3,954,579, 3,959,095, 4,061,550, 4,075,068). The buffer compartment is defined by a cation- and an anion-permeable membrane, with the purpose of this neutral compartment being to retain the production of high quality caustic in the catholyte and optimize the production of chlorine gas from the anolyte.
The placement of the electrodes within the chlor-alkali cell varies with the design of the unit and its application in the industrial or consumer sectors. The distance between electrodes is a function of design. It is arbitrary whether the electrodes are placed vertically (U.S. Pat. No. 4,100,050) or horizontally (U.S. Pat. No. 3,976,550) and this choice has previously not been a functional parameter. With a current applied across the anode and cathode within a conducting solution, the unit will increase in temperature such that depending on design, a separate heat exchanger may be required (U.S. Pat. No. 3,669,857).
Within the general sector of the application of chlor-alkali cells focussing on the generation of chlorine gas, there exists a division of on-site application which utilize the generated chlorine gas as a disinfectant when mixed with water. The active biocide is hypochlorous acid. These on-site chlorine generating units retain certain additional design modifications which facilitate operation and convenience.
One of these divisions in the use of the chlor-alkali cell is the use of the chlorine generator within swimming pool applications. Several varieties of swimming pool chlorine generators exist and include: the direct addition of salt to the pool water and the passing of this saline pool water over an anode and cathode within a non-partitioned electrolytic unit (U.S. Pat. Nos. 2,887,444, 3,378,479); the generation of chlorine with a non-partitioned cell using a hydrochloric acid electrolyte and the subsequent addition of the electrolyte directly to the swimming pool for chlorination and pH control purposes (U.S. Pat. No. 3,351,542); the generation of a chlorine gas and hypochlorous acid anolyte solution from a sodium chloride electrolyte using a membrane cell with the direct application of the anolyte to the pool for disinfection purposes and the altered addition of the caustic solution to either the pool or to drain for pH control of the pool (U.S. Pat. No. 3,669,857); and the generation of chlorine gas using a conventional type membrane cell with the chlorine gas administered using an aspirator mixing unit to combine the chlorine gas with the pool water plus retaining a means to continuously withdraw abrasive caustic from the unit (U.S. Pat. No. 4,097,356).
In addition to the application of the on-site generation of chlorine gas for swimming pools disinfection, a variety of chemical treatments have been administered to the pool water to optimize the effectiveness of the available chlorine present in the pool. These means are aimed at controlling the alkalinity, acidity and hence pH of the pool. These means include the administration of buffering chemicals to the pool water such as tripolyphosphate, sodium biborate, sodium dibasic phosphate, sodium pyrophosphate, and sodium hexametaphosphate to facilitate pH control (U.S. Pat. No. 2,887,444), or the use of solid chemicals such as calcium carbonate within the chlorinated stream to control pH (U.S. Pat. No. 3,361,663) with the subsequent addition of calcium hardness and bicarbonate alkalinity to the pool water.
By utilization of the present invention, the on-site nascent generation of chlorine gas in swimming pool disinfection is possible such that the sole addition of chlorine gas is provided without the withdrawal of corrosive, potentially hazardous materials such as caustic from the electrolytic unit and without the administration of additional chemicals to the pool water. Additionally, no central membrane is required, thus avoiding the inevitable problem of membrane foul-out. As is hereinafter disclosed, the present invention retains a closed electrolytic chlorine generator which does not continuously generate abrasive caustic solutions, does not require a membrane, nor does it require other chemical additives to be placed directly into the swimming pool.